There is known a differential gear device including: (a) a differential casing rotatable about an axis; and (b) a pair of side gears which are disposed within the differential casing such that the side gears are rotatable about the above-indicated axis, such that the side gears are opposed to each other, and such that a rotary motion of the differential casing about the above-indicated axis is transmitted to the side gears through pinion gears, so as to permit differential rotary motions of the side gears. The differential casing has a pair of bearing portions which extend from its respective inner surfaces toward back surfaces of the respective side gears and which have respective annular seating surfaces which are coaxial with the respective side gears and which receive axial loads from the side gears. JP-2017-116035A discloses an example of such a differential gear device. In the differential gear device disclosed in this publication, a lubricant oil in the differential casing is splashed up by the side gears, and a differential ring gear attached to the differential casing, to lubricate various parts of the differential gear device. Further, annular seating surfaces of the differential casing, which are provided for the respective side gears, have lubricant grooves formed so as to extend from their inner edges to their outer edges, while back surfaces of the side gears have lubricant scattering preventive portions at their outer edges, to hold the lubricant oil between the seating surfaces and the back surfaces of the side gears, for lubricating washers interposed therebetween, for instance. In addition, inner circumferential surfaces of bores which are formed through the differential casing and through which rotary shafts connected to the side gears extend have spiral grooves through which the lubricant oil outside the differential casing is introduced into the differential casing and directed in between the seating surfaces and the back surfaces of the side gears.
However, the differential gear device constructed as described above has a problem of a low resistance to seizure due to shortage of an amount of the lubricant oil supplied in between the back surfaces of the side gears and the seating surfaces when the differential gear device is placed in a differential state in which one of the two side gears is held stationary and does not function to splash up the lubricant oil within the differential casing. This problem is serious particularly where a level of the lubricant oil within the differential casing is lowered to reduce a power loss due to the splashing of the lubricant oil. The problem is also serious where the lubricant oil has a low temperature, and has a high degree of viscosity and an accordingly low degree of fluidity at a low temperature of the lubricant oil.